1. Field of the Invention
The present invention relates to a motor control method and motor control apparatus for controlling a plurality of motors, and an image forming apparatus.
2. Related Background Art
A conventional motor control apparatus, which is required to synchronously rotate and stop a plurality of motors, normally uses motors which are controlled by an identical control method and have relatively small variations in rotation characteristics due to their individual differences, for example, pulse motors, DC motors, and the like.
For example, in an image forming apparatus using electrophotography, a motor for driving a photosensitive element as an image carrier and a motor for driving a convey means for conveying a transfer element such as a recording sheet or the like to the transfer position of the photosensitive element must be synchronously rotated not only during transfer for transferring a toner image on the photosensitive element onto the transfer element but also at the beginning or end (stop) of rotation of the photosensitive element. In such case, since motors, which are controlled by an identical control method and have relatively small variations in rotation characteristics due to their individual differences are used, synchronous control of these motors is relatively easy.
On the other hand, especially in an image forming apparatus using electrophotography, a motor with high rotation precision is required to drive the photosensitive element or transfer element so as to form a high-resolution image. To meet such a requirement, the following technique has been proposed. For example, a vibration wave motor that is a vibration type motor is used as one of the motors for driving the photosensitive element or transfer element together with other motors controlled by a different control method.
As proposed by Japanese Patent Application Laid-Open No. 58-14682, a vibration-type motor excites a plurality of vibrations at frequencies beyond the audible range in a vibration member, and obtains a driving force by synthesizing these vibrations. As for its driving performance, the vibration type motor realizes stable rotation at a constant speed, as described in detail in Japanese Patent Application Laid-Open Nos. 63-1379, 60-176470, 59-204477, and the like.
However, the vibration type motor suffers variations in its rotation characteristics due to individual differences in the motors, and the driving frequency and voltage and the pulse width of the driving voltage must be periodically adjusted in correspondence with a signal output from a speed detection means, such as an encoder signal, so as to maintain stable rotation.
When a plurality of vibration-type motors or a vibration-type motor and a motor controlled by a different control method are synchronously controlled, these motors engage in synchronous rotation along different speed transition curves depending on the individual differences in their rotation characteristics or different control methods for the individual motors, especially at the beginning and stopping of rotation.
During this transition period, since a relative speed is produced among the elements to be driven by these motors, friction is produced among these elements to be driven, resulting in wears or damage to the elements to be driven.
More specifically, the time for start up operation commonly varies due to variations of the motors, as shown in FIG. 10. In case of FIG. 10, the times required for the motor to reach a target speed have a time difference T2-T1 between two motors, and the difference in the total moving amount corresponds to the area bounded by the solid curve and one-dashed chain curve. Hence, the speed difference between the motors increases upon start up unless the start up operations of the motors are controlled, and may promote wear of the elements to be driven.